LATEST YELLOWSTONE VOLCANISM: ROARING MOUNTAIN RHYOLITES, YELLOWSTONE VOLCANIC FIELD, WYOMING

Yellowstone Volcanic Field (YVF) has been the site of 3 major caldera-forming eruptions over the past 2 Ma with the last silicic intracaldera volcanism occurring at 70 ka and the last basaltic extrusion at 209 ka. Coeval with intracaldera volcanism are the Obsidian Creek and Roaring Mountain extracaldera rhyolites, located just north of the Yellowstone caldera in the Norris-Mammoth Corridor. Initially, these extracaldera rhyolites were believed to have originated from small, short-lived, and unrelated magma batches. Recently it has been shown that the extracaldera lavas are derived from an evolving magma system with a lifespan of up to ~400ka.

New data indicates that all three lava flows from the younger Roaring Mountain Member are high-temperature fayalite rhyolites evolved from the same magma system. Linear Harker diagram trends link Obsidian Cliff (OC) as the silicic end-member of a mixing line involving mingled lava Crystal Spring (CS) and Swan Lake Flat basalt. CS mingled lavas contain as much as 20% mafic material, with mafic enclaves decreasing towards OC to the south. The entire OC flow is a homogeneous, aphyric obsidian with no evidence of mingling. These relationships suggest that OC and CS were produced by the same eruption. The extensive Gibbon River (GR) rhyolite is more chemically complex with at least three distinct lavas. Quartz and feldspar temperatures also vary widely across the GR flow (800-975°C). The majority of the flow is a two-feldspar rhyolite. A slightly more primitive rhyolite with Na-rich sanidine is located at the north end of the flow. A third rhyolite along the SW flow edge is a slightly evolved mixture of the other two GR rhyolites.

New ⁴⁰Ar/³⁹Ar geochronology yielded the youngest ages to date from the YVF. Step-heating analyses of fresh obsidian provide three plateau ages with a weighted mean of 59.15 ± 1.97 ka for the eruption of OC (and inferred CS eruption age). Single-grain sanidine laser fusions from GR provide an inverse isochron eruption age of 55.6 ± 2.7 ka. These new ages and chemistries indicate that extracaldera volcanism is younger and the magma system more complex than previously thought. The young age for the CS mingled lava also indicates continued basaltic underplating of the YVF to as recent as 59 ka.